Crosslinking of the surface receptors on resting murine B cells (either IgD or IgM) with antibodies directed to the receptors results in a series of biochemical events. These include hydrolysis of phosphatidyl inositol trisphosphate with the resulting production of inositol trisphosphate and diacylgycerol. Recently it has been found that in addition to this pathway a tyrosine kinase is also activated. When the IgD receptor is crosslinked using both mitogenic and nonmitogenic antibodies there is a rapid phosphorylation of a 69Kd substrate, this substrate is dephosphorylated about 1 minute after the initiation of the signal. We are attempting to identify this substrate as to date this phosphorylation is the only difference seen when comparing stimulating anti-IgD to anti-IgM antibodies.